BLEACHING. This term implies the whitening of objects or depriving them of colour. In its industrial application, how ever, the term signifies not only the removal of the natural colour but other impurities from cotton, linen, wool, silk and other fibres, paper-pulp, bees' wax, and some oils. The removal of colours which have been acquired by dyeing textiles is called "stripping," and the local bleaching of parts of a coloured piece in printing silk or calico is termed "discharging." Flour, sugar, and many other substances are decolourized in the course of their prepara tion for the market.
The art of bleaching textile fabrics, such as linen and wool, is of great antiquity and seems to have been familiar to all the civilizations of the world. Egyptians, Phoenicians, Greeks, and Romans are known to have produced white linen goods, but little is known of the methods which they employed. Pliny refers to the use of plants and the ashes of plants, mentioning especially the "Struthium," which was probably the plant Saponaria officinalis. Information about bleaching in Europe was difficult to obtain until after the last of the crusades, when the Dutch es tablished a great reputation as linen bleachers, and retained almost a monopoly of that industry down to the middle of the i8th century. Bleaching grounds are said to have existed near Manchester, England, as early as 1322, and there was a bleach works at Southwark, near London, in the middle of the i7th century, but the bulk of the brown linen manufactured in Scot land was sent to Holland to be bleached. It was sent to the neigh bourhood of Haarlem in the month of March, and was not returned till the end of October. In 1728 the Scottish Board of Manufactures accepted a proposal made by James Adair of Bel fast to establish a bleachfield in Galloway, and devoted f 2,000 as premiums for the establishment of bleachfields throughout the country. With the assistance of the same Board, R. Holden in 1732 set up a bleachfield at Pitkerro, near Dundee, where he used a method of bleaching with kelp.
The British bleachers (who were sometimes called whitsters) appear to have been taught, or to have copied, the methods of the Dutch, which consisted in steeping the linen in alkaline lyes for several days, washing it clean, and spreading it on the grass for some weeks. These processes, called respectively bucking and crofting, were repeated alternatively for five or six times. The cloth was then steeped in sour milk or butter milk for some days, washed clean, and again crofted. The processes were repeated until the material had acquired the requisite whiteness. In 1736 an act was passed in England which permitted cotton to enter into the composition of cloth and this gradually resulted in the manufacture of larger quantities of material, and consequently a greater demand for bleaching.
A very effective means of shortening the time of bleaching was the substitution of sour milk by dilute sulphuric acid. This method of "souring" was suggested by Dr. Francis Home, of Edin burgh, who in 1756 published a scientific work on bleaching. It was found that water acidulated with sulphuric acid would bring about an effect in 12 to 24 hours which could be obtained only of ter steeping for six to eight weeks in sour milk. This reduc tion in the time of bleaching rendered it possible for the mer chant to dispose of goods so much sooner and consequently to trade with less capital.
A far reaching change in the character of the bleaching indus try commenced in the year 1787. In that year chlorine, discovered in 1773 by Scheele, was used in the works of Macgregor, of Glasgow, where its application had been suggested by James Watt, who was the son-in-law of the owner. Watt had gained his infor mation about oxygenated muriatic acid, as chlorine was then called, from Berthollet, who was the first to record the effect of the gas in bleaching the natural colour of linen. In a paper read before the Academy of Sciences at Paris, in 1785, and published in the Journal de Physique (vol. xxvi. p. 3 2 5) Berthollet men tioned that he had found the gas to answer perfectly in the bleach ing of cloth. Further work was recorded in the Journal de Physique (1786). Thomas Henry, of Manchester, was the first to bleach with chlorine in the Manchester district. At this period, the bleacher made his own chlorine and exposed the fabric in chambers to the action of the gas, or steeped them in an aqueous solution. The process was inconvenient, disagreeable and, worst of all, detrimental to the health of the workers, so it was not sur prising that, in spite of the rapid bleaching action, the method did not gain great favour. Considerable improvement was effected by the introduction, in 1792, of eau de Javel, which was made near Paris by absorbing chlorine in a solution of potash (one part) in water (eight parts) until effervescence began. In Charles Tennant, of Glasgow, successfully prepared bleaching powder by passing chlorine over lime and was able to supply the bleacher with a reagent in solid form, which contained up to one-third of its weight of available chlorine. The pleasant fields, where the industrious farmer could spread his cloth in the sum mer sun, were soon covered with buildings, and that part of the process which was formerly left to light, air, and moisture was carried out by the aid of calcium hypochlorite—the active con stituent of bleaching powder solution. Attempts to replace bleach ing powder have met with little success, except where cheap elec trical power is available for the manufacture of sodium hypo chlorite from common salt. Improvements in the bucking process were most marked about 1823, when sodium carbonate was sub stituted for potash. (See ALKALI MANUFACTURE.)